Phlogopite refers to micas at the magnesian end (Mg/(Mg+Fe) > 0.67) of the biotite-phlogopite solid solution series.  Phlogopite has many of the optical characteristics of biotite, with the most important difference being that phlogopites are less strongly coloured (i.e., the pleochroic colours are not as intense).  A and B show views of a phlogopite xenocryst in a kimberlite, with the stage rotated 90 degrees between the two views.  The rather pale brown pleochroic colours of the main body of the grain are evident, with maximum absorption when the cleavage traces are oriented parallel to the polarizer (EW in these images).  Overgrowth of new phlogopite from the kimberlite has resulted in rims and zones of strongly reddish brown  phlogopite that has its pleochroic scheme reversed (i.e., strongest absorption occurs when traces of the cleavage are perpendicular to the polarizer).  This is caused by the presence of ferric iron in the tetrahedral site, and is called tetraferriphlogopite.  The third-order interference colours of the phlogopite are obvious in C, and it is also clear that the rims have lower interference colours. In D, a group of phlogopites of various orientations (with slightly different pleochroic colours) occur with calcite.  A, B, C from the Wesselton Mine kimberlite, South Africa, and D from phlogopite-bearing  marble from the Grenville Province of eastern Ontario.  A, B and C are 5.5 mm across, D is 2.2 mm across.  A, B and D ppl, C x-nicols.